It is previously known to use resolvers as position transducers in industrial robots. To achieve a desired high accuracy of a position value obtained with the aid of such a resolver, the resolver is usually arranged such that the operating range of the axis covers a plurality of revolutions of the resolver. This can be achieved, for example, by mounting the resolver on the shaft of a motor which drives the robot axis via a gear. However, this solution has the drawback that the resolver signal does not give an unambiguous indication of the position of the axis. Therefore, the equipment must be supplemented with electronic members in the form of counters, memories, etc., which continuously keep track of within which resolver revolution the axis is at the moment. These members must be reset when putting a robot into service and thereafter after each voltage drop out or other switch-off of the voltage supply of the robot. These so-called synchronization operations require time and manual effort. Therefore, it is also known with so-called absolute-measuring position transducer systems, i.e. systems which provide an unambiguous—and accurate—determination of the position of a robot axis without requiring any synchronization operations. It has been proposed to design such a system with two resolvers for each robot axis. One resolver is arranged so as to rotate a plurality of revolutions when the robot axis moves between the limits of its operating range, thus making possible an accurate determination of the position. The other resolver is arranged so as to rotate less than one revolution when the robot axis moves between the limits of its operating range, and with the aid of this resolver an unambiguous determination may be obtained as to within which revolution the first resolver is situated.
Thus, by combining the output signals of the two resolvers, an unambiguous determination of the position of the robot axis may be obtained. However, this solution requires two resolvers per robot axis and is therefore complicated and expensive. In such a position transducer equipment using two resolvers per robot axis, these have been connected to a control system common to all robot axes, which system then comprises supply and sensing members for the resolvers. Since each resolver has three windings, an extensive cable arrangement between the resolvers and the control system is then required. Such a cable arrangement will be expensive and bulky. This is particularly the case with absolute-measuring position transducer systems having two resolvers per robot axis. From EP-A-177 901 it is previously known to provide an absolute-measuring position transducer system for an industrial robot by arranging for each robot axis two transducers, i.e. a resolver and a pulse transducer with associated circuits for sensing the direction of movement and counting the aggregate number of revolutions of the axis. In normal operation, the number of whole revolutions for each axis, obtained from the sensing circuits of the pulse transducer, is combined in a calculation circuit with the angular position within each revolution, which is obtained from the resolver of the same axis. In case of drop out of the supply voltage, the pulse transducer with associated reading circuits is supplied from a battery and stores a value which correctly indicates the number of whole revolutions of the axis independently of movements of the robot during the voltage drop out period. The pulse transducer may be fed from an oscillator with a low on/off ratio in order to reduce the power consumption during the battery supply interval. Thus, in this known system, the resolver with its high current consumption is not used during the battery feed interval, but the resolver is supplement with another type of position transducer, a pulse transducer, which in itself has a low power consumption. In the system known from EP-A1-0 177 901, there are thus required for each axis a resolver and a transducer with associated sensing and storage circuits. A typical industrial robot has six axes, and the absolute-measuring function is thus obtained at the cost of a considerable complication and increase in price of the robot. Further, an absolute-measuring position transducer system for an industrial robot, which determines the axis positions with high accuracy but which has a relatively simple and inexpensive design with respect to both the transducer units and the required cable arrangement is known from EP-A1-0 406 740.